Imprint identification system

ABSTRACT

Images of imprints, e.g. made by items of footwear, are displayed on a computer screen. Any identification features A-D which are present in the image are tagged and the length of all the lines 1-6 joining the tagged features are calculated and the distance information is added to a database together with the images themselves. The polygons defined by the lines 1-4 are independent of the positioning or orientation of the image and are used to retrieve images likely to originate from the same article by applying search criteria which retrieves records containing distances falling within selected bands.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a method of identifying footwear or otherimpressions left, for example, at places where crimes have beencommitted. Although the following description refers specifically tofootwear it will be appreciated that the method is also applicable toimpressions left by vehicle tires, parts of the body or tools forexample.

BACKGROUND

WO 97/28 513 discloses a system in which a digital image of a footprintis captured, displayed on a computer screen, cropped to a predeterminedsize, and the rectangular co-ordinates of any unique identificationfeatures are recorded and stored in a database. The stored co-ordinatescan then be compared to identify any similar sets of co-ordinates whichare likely to originate from the same article. In order to reduceregistration errors care must be exercised in the positioning of theimage. In addition, in order to take account of any registration errorsmultiple comparisons are made with the sets of co-ordinates beingincremented or decremented between comparisons.

The present invention seeks to provide an inventive improvement on theearlier system.

SUMMARY OF THE INVENTION

The present invention proposes a method of imprint identification,comprising:

-   -   obtaining, with a predetermined reproduction ratio, an image        from an imprint produced by an article; and recording the        co-ordinates of identification features present in the image;        -   characterised by    -   calculating the distances between such co-ordinates;    -   storing a record of the distances thereby obtained in a database        containing a number of similar records; and    -   comparing the distance information of the stored records to        identify records likely to have been derived from the same        article.

If the lines joining the co-ordinates are displayed it will be foundthat they produce a polygon formed of a number of triangles. Each suchpolygon will be unique to the particular article from which the imagewas obtained. Moreover, the shape of the polygon as defined by thespatial information (distances) will not change with time and will beindependent of the positioning and orientation of the image. Thus, bycomparing the distance records, imprints likely to originate from thesame article can quickly be identified without the need for multiplecomparisons. The requirement for accurate positioning of the image istherefore eliminated and the retrieval speed is greatly improved.

The sets or distances are preferably selected according to definedsearch parameters covering a range of distances. By adjusting theparameters the number or recovered records can be changed. Thus, byprogressively restricting the search parameters the number of recordscan be progressively reduced until only records likely to originate fromthe same article are identified.

The retrieved records may be displayed in various ways, but it isgenerally convenient to display the records on separate rows of a tablewith the distance information arranged in columns, preferably innumerical order.

The database preferably includes the images themselves so that theimages can be downloaded and visually compared when required. Thus,images possessing similar polygons but different tread patterns canquickly be eliminated since they obviously originate from differentarticles.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred totherein are included by way of non-limiting example in order toillustrate how the invention may be put into practice. In the drawings:

FIG. 1 is an image of a shoe imprint as used in the method of theinvention, and

FIG. 2 is a spread-sheet table used to display distance data taken froma number of such images.

DETAILED DESCRIPTION OF THE DRAWINGS

A digital image is obtained from a footprint found at the scene of acrime. The image is taken in a fixed reproduction ratio in accordancewith any of the techniques described in WO 97/28 513. The image isimmediately written to a compact disc (CD-ROM or DVD) in a bitmap fileformat for permanent storage so that it can be retrieved for future use,e.g. for production in evidence in criminal proceedings. Examples ofsuitable bitmap file formats are Windows BMP, TIFF and TGA. Tofacilitate accurate identification of the image, identification data isincorporated into the header of the bitmap file, e.g. the date, time andlocation where the image was taken. Furthermore, in order to eliminatethe possibility of tampering a non-alterable duplicate copy of the image(known as a watermark) is recorded on the CD. The duplicate image cannotnormally be displayed except by using secure retrieval software whichenables the two images to be overlayed and compared such that and anydiscrepancy between the two images is highlighted.

In addition to permanent storage the images are subjected to furtherprocessing in a way which will now be described. The image is displayedon a computer screen and cropped if necessary to remove any unwantedmargins around the footprint. The display resolution is initially set sothat the full image is displayed, as shown in FIG. 1. Any characterisingmarks present in the image are then tagged to record their rectangular(x, y) coordinates by positioning a cursor anywhere on the area of amark and clicking with the computer mouse. The kind of features whichare recorded generally fall into two categories:

-   -   i) Manufacturing (moulding) defects.    -   ii) Damage caused through wear, such as physical damage (e.g.        cuts), inclusions (e.g. pieces of flint or metal), or areas of        heavy general wear due to the particular gait of the wearer.

In order to allow more detailed examination of the image andidentification of characterising features as well as facilitating moreaccurate positioning of the cursor within a characterising mark theimage can be zoomed by up to 1,600% to increase the display resolutionof any desired area. It will however be appreciated that magnifying theimage in this way does not change the underlying resolution of thestored image.

Although the cursor can be manually positioned at the centre of a markmore accurate and reproducible results can be achieved by utilisingsoftware algorithms which calculate the geometrical centre of thecharacterising marks. Software sub-routines can auto-trace the contrastboundary of the mark, calculate the geometrical centre of the tracedarea, and then auto-align the cursor with the calculated point. Accuratepositioning is therefore possible even with irregular areas of damage,e.g. diffuse areas of general wear.

Different kinds of characterising feature can be tagged with particularidentifying symbols (e.g. circle, cross, star etc.). The ability todistinguish between different kinds of feature further enhances thediscrimination of the system.

Unless the footwear is virtually new, at least three characterisingfeatures will normally be present. In the example shown in FIG. 1 foursuch features are identified, labelled A to D. When all the featureshave been tagged, the computer calculates the distances between all ofthe tagged points. In the case of the image shown in FIG. 1, the fourpoints produce six distance values which are represented by the lines 1to 6 in the drawing. These lines are not necessarily displayed to theuser but they are shown in the drawing to illustrate the unique polygonwhich they define.

It will be appreciated that the distance values and the shape of thepolygon will be the same irrespective of the positioning of the image,and in fact, even the orientation of the image will not alter theresulting distances. Only the base resolution of the image (e.g. thenumber of pixels per cm) will affect the distances, but this iseliminated by ensuring that all images are obtained with a knownreproduction ratio (conveniently 1:1).

The manual tags and polygons are stored as separate files appended tothe original image file, and the calculated distance values are added toa central database. Each new set of readings creates a new record in thedatabase. The image is also uploaded to the database together with theappended tag files.

By using appropriate search criteria it is possible to retrieve recordswhich have similar distance values. Initially it will generally bedesirable to use broad search criteria, e.g. all distances fallingwithin a small number of defined distance bands. The retrieved recordsare then conveniently displayed in spread-sheet format, as shown in FIG.2. Each row of the spread-sheet corresponds to a different record. Thesearch criteria can be progressively narrowed to reduce the number ofrecords until only those likely to originate from the same item offootwear are displayed. By displaying the distance values falling withindifferent bands in different colours it is possible to quickly identifythe records most likely to be of interest. For example, in the drawingthe values having the suffix “R” will be displayed in red, those havingthe suffix “Y” would be yellow, and “B” would be blue. Thus, there areonly two records (rows 5 and 6) which contain values falling within allthree specified bands, and these records can be selected to allowexamination of the records and their associated files in more detail.

As the item of footwear ages additional characterising features will beadded so that the number of distance values obtained from an image willtend to increase. It is important to appreciate however that thedistances between existing features will not change so that sufficientcommon distance values will still be present to allow accurate retrievalof related records. It is of course possible that characterising markswill be lost as the footwear ages and shallow features wear away, butagain there will generally be a sufficient number of common valuesremaining to allow reliable retrieval of related records.

When records which might be related have been identified the originalbitmap images and tag files can be downloaded for detailed examination.Clearly, any images having similar distance values but different treadpatterns can be eliminated at this stage. A manual examination willgenerally confirm whether the footprints originated from the same itemof footwear. The tags and polygons can be superimposed on any image toassist manual comparison and identification of related images.

A second database can be set up as described in the aforementionedpatent specification, containing similar data obtained by scanning thefootwear of known suspects whilst they are held in custody. Again, theimages are added to the second database with identification datarecorded in the bitmap file header. Such details include date, time andlocation of image recording, the name and collar number of the officerwho made the recording, the station code, suspects name, custody number,nominal number, shoe make, model, size, offence, and (if desired) otherdetails pertaining to the offence in free text form. It is thuspossible, by searching and comparing data from both databases, to linkindividual offenders to the scenes of crimes at which footprints wereretrieved.

In summary therefore, by comparing the distance records, imprints likelyto originate from the same article can quickly be identified. Therequirement for accurate positioning of the image is eliminated and theretrieval speed is greatly improved. The image retrieval process doesnot affect other substances which might be present on a suspectsfootwear so that it can then be examined for forensic evidence. There isalso a significant reduction in running costs compared with existingimage retrieval and storage systems.

The items of information which can be added to the database records canbe extended to include additional unique identifying data with theobject of further improving the accuracy and reliability of the dataretrieval. For example, in addition to the co-ordinates of the taggedareas the database can include the points of intersection of the linesjoining the tagged points. Furthermore, the polygon can be linked to aspecific moulding pattern by recording the co-ordinates of the points ofintersection between the lines of the polygon and features of the solepattern. Even greater discrimination can be achieved by recording theangle of incidence at the said points of intersection.

Although this example refers to imprints obtained from items of footwearit will be appreciated that a similar system can be used to compareunique areas of damage or characteristic defects in other articles suchas vehicle tires or tools. Similarly, by tagging known reference pointsIn imprints left by body parts which vary in shape between individuals,e.g. ear imprints, similar sets of distance information can be derivedwhich can be used to identity imprints originating from the sameindividual.

It will be appreciated that the features disclosed herein may be presentin any feasible combination. Whilst the above description lays emphasison those areas which, in combination, are believed to be new, protectionis claimed for any inventive combination of the features disclosedherein.

1. A method of imprint identification, comprising: obtaining, with apredetermined reproduction ratio, an image from an imprint produced byan article; and recording the co-ordinates of identification featurespresent in the image; characterised by calculating the distances betweensuch co-ordinates; storing a record of the distances thereby obtained ina database containing a number of records similarly obtained from otherimprints; and comparing the distance information of the stored recordsto identify records likely to have been derived from the same article.2. A method of imprint identification according to claim 1, in which therecords are retrieved from the database using search parameters whichcover a plurality of defined distance bands.
 3. A method of imprintidentification according to claim 2, in which the records are displayedon separate rows of a table with the distances arranged in columns.
 4. Amethod of imprint identification according to claim 3, in which thedistances are displayed in ascending or descending numerical order.
 5. Amethod of imprint identification according to claim 3, in whichdistances which fall within the search parameters are visuallydistinguished.
 6. A method of imprint identification according to claim1, in which the database includes downloadable images of the imprints.7. A method of imprint identification according to claim 1, in which thedatabase contains the co-ordinates of the identification features fromwhich the distance information is derived.
 8. A method of imprintidentification according to claim 1, in which the database contains afurther group of records containing distance information obtaineddirectly from articles.
 9. A method of imprint identification,comprising: obtaining, with a predetermined reproduction ratio, an imagefrom an imprint produced by an article; and recording the co-ordinatesof identification features present in the image; characterised bycalculating the distances between such co-ordinates; storing a record ofthe distances thereby obtained in a database; repeating the steps ofobtaining an image from another imprint, recording the coordinates ofidentification features present in the image, calculating the distancesbetween such co-ordinates and storing a record of the distances therebyobtained so that the database contains stored records from differentimprints; and comparing the distance information of the stored recordsto identify records likely to have been derived from the same article.10. A method of imprint identification according to claim 9, in whichthe records are retrieved from the database using search parameterswhich cover a plurality of defined distance bands.
 11. A method ofimprint identification according to claim 10, in which the records aredisplayed on separate rows of a table with the distances arranged incolumns.
 12. A method of imprint identification according to claim 11,in which the distances are displayed in ascending or descendingnumerical order.
 13. A method of imprint identification according toclaim 11, in which distances which fall within the search parameters arevisually distinguished.
 14. A method of imprint identification accordingto claim 9, in which the database includes downloadable images of theimprints.
 15. A method of imprint identification according to claim 9,in which the database contains co-ordinates of the identificationfeatures from which the distance information is derived.
 16. A method ofimprint identification according to claim 9, in which the databasecontains a further group of records containing distance informationobtained directly from articles.